The frequent occurrence of tuberculosis in patients with diabetes mellitus is well known. The object of this study was to find the incidence of undiscovered diabetes mellitus in patients with known tuberculosis. A total of 118 sanatorium patients over the age of 15 years were tested with one-hour postcibal blood sugar determinations in February 1961. True blood sugars were used. Nine patients had a value of 148 mg. % or higher. Glucose tolerance tests were carried out on eight of the nine patients with levels of 148 mg. % or higher. Five of these were shown to have diabetes. The incidence of undiscovered diabetes was 4.2% of the patients over 15 years of age in the sanatorium. It is predicted from the shape of the frequency distribution curve that this figure could be as high as 10% if the glucose tolerance tests were done at a level of 130 mg. % or higher.Routine one-hour p.c. blood sugar tests are recommended in all tuberculous patients and glucose tolerance tests in all of those patients with a level of 130 mg. % or higher.